(A) Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a light-emitting device having a ring-shaped photonic crystal structure configured to improve the light-extraction efficiency and to control the collimation property of the light beams.
(B) Description of the Related Art
The lighting efficiency of the solid-state semiconductor light-emitting devices such as the light-emitting diode (LED) is restricted to the total reflection at the interface between the internal semiconductor material and the external material such as the air or the epoxy, and only a small percentage of the total light generated in the device is emitted to the external environment.
FIG. 1 and FIG. 2 illustrate a conventional light-emitting diode 300, disclosed in U.S. Pat. No. 7,098,589. The light-emitting diode 300 includes a substrate 320 and multi-layer stack 322 positioned on the substrate 320. The multi-layer stack 322 includes a gallium nitride layer 324 having a plurality of circular openings 350. In particular, the technique disclosed in U.S. Pat. No. 7,098,589 uses the circular opening 350 to prevent the total reflection at the interface so as to improve the light-extraction efficiency. In general, a larger opening corresponds to higher light-extraction efficiency. However, a large opening negatively influences the current distribution of the light-emitting device and the lighting efficiency of the light-emitting layer, and decreases the whole light-extraction efficiency.